Operation Manual – IPv4 RoutingH3C S7500E Series Ethernet Switches Chapter 5 BGP Configuration5-5V. KeepaliveKeepalive messages are sent between peers to maintain connectivity. Its formatcontains only the message header.VI. Route-refreshA route-refresh message is sent to a peer to request the resending of the specifiedaddress family routing information. Its format is shown below:Figure 5-5 BGP Route-refresh message formatAFI: Address Family Identifier.Res: Reserved. Set to 0.SAFI: Subsequent Address Family Identifier.5.1.2 BGP Path AttributesI. Classification of path attributesPath attributes fall into four categories:z Well-known mandatory: Must be recognized by all BGP routers and must beincluded in every update message. Routing information error occurs without thisattribute.z Well-known discretionary: Can be recognized by all BGP routers and optional tobe included in every update message as needed.z Optional transitive: Transitive attribute between ASs. A BGP router not supportingthis attribute can still receive routes with this attribute and advertise them to otherpeers.z Optional non-transitive: If a BGP router does not support this attribute, it will notadvertise routes with this attribute.The usage of each BGP path attribute is described in the following table.Table 5-1 Usage of BGP path attributesName CategoryORIGIN Well-known mandatoryAS_PATH Well-known mandatoryNEXT_HOP Well-known mandatoryLOCAL_PREF Well-known discretionaryATOMIC_AGGREGATE Well-known discretionary
